'73 JMP 50 with no voltage selector switch or cutout plus cap question

Discussion in 'Let's Talk Vintage' started by gushtone, May 4, 2020.

  1. gushtone

    gushtone New Member

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    Hi all,

    New to the forum. My friend has a '73 JMP 50 with no voltage selector switch, no cutout for one, and the Mains wire/power cord has a standard US looking clip in a small hole, no larger Bulgin style cutout. Pardon my ignorance, but is this a "intended for the US market model"? Otherwise it looks completely straight.

    Thanks!

    Peace...
     
    Last edited: May 5, 2020
  2. gushtone

    gushtone New Member

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    Also, why do some pre-circuit board JMP 50's have two large caps on the other side of the big black thing on the turret board down towards the power Tubes, and some do not? Apologies again for my ignorance, Any help would be greatly appreciated.

    Thnaks.
    Peace,,,
     
  3. CoyotesGator

    CoyotesGator Well-Known Member

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    Pictures please, some of us can’t read.
     
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  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Well I've never seen one without a mains power switch.
    But you can post pictures after a short period in the forum. That would be interesting to see.

    Also, welcome to the forum Gushtone. :cheers:
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    In the earlier models (some models) there were filter caps mounted inside the chassis.
     
  6. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Are you referring to a lack of a mains voltage selector on the BACK panel? That's standard for US market amps, which came with a permanently attached power cord as well. The Bulgin power socket usually means "Made for anywhere but the US or Canada".

    It WILL always have a mains power switch, and a standby switch as well. If either is missing, then someone has seriously monkeyed with the amp.

    Now, the POLARITY switch, that disappeared at some point as it's not necessary when using a grounded 3 prong outlet. If the amp has a polarity switch, the best thing
    to do is remove what we call the "death capacitor" (death cap) and disable the polarity switch, or repurpose it for some switchable mod in the amp.
     
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  7. neikeel

    neikeel Well-Known Member

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    Sounds like Export USA model (no selector and fixed mains cord)
    You select for input voltage by a turret strip inside the amp.
    The caps are the snubber caps (to minimise diode switching noise) the black thing on the circuit board is the rectifier block.
    Pics always help.
     
  8. gushtone

    gushtone New Member

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    Thanks all. My bad, not mains switch, mains voltage selector switch, so this makes sense. And yes, regarding the caps I was referring to, some JMP 50’s have two large caps on the other side of the rectifier block (snubber caps? Thanks!). So sometimes these were installed under the Turret board? Or not used at all?
    I will try to post a pic later.

    thanks for your help!
     
  9. neikeel

    neikeel Well-Known Member

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    Ex factory the snubbers on the board appeared 1969 and seen til 73.
    No, snubbers never fitted under the board. Earlier amps had a single 0.05uF red Hunts or Erie labelled cap off the HT fuse holder to ground for similar purpose and also helps as anti- pop cap when you switch off.
     
  10. Marshallhead

    Marshallhead Well-Known Member

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    If it’s a Lead & Bass 50w then the UK models also came with a captive mains lead.
     
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